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Interpretive Summary: The fungus Sclerotinia trifoliorum is a pathogen of chickpea and other cool season legumes, and it produces spores of two sizes. The large spores give rise to strains that are self-fertile and produce large and small spores again, but the small spores give rise to strains that are self sterile and cannot produce any spores. This phenomenon is called unidirectional mating type switching. The mechanisms of unidirectional mating type switching in S. trifoliorum are not understood. The present study, comparing DNA sequences in the DNA region (MAT locus) that controls mating and spore production, found four main mating type genes (MAT1-1-5, MAT1-1-1, MAT1-2-1 and MAT1-2-4) in the self-fertile strain derived from large spores. In contrast, this DNA region in the self-sterile strain derived from small spores is 2891-bp shorter than that of the self-fertile strains. Within the missing DNA are the entire MAT1-2-1 and MAT1-2-4 genes. Thus the self-sterile strains do not contain the MAT1-2 genes and that is why they are self-sterile. Meanwhile, a146-bp DNA sequence was found repeated twice and the two repeats flank the deleted region in the self-fertile strains. The two repeat sequences were believed to be responsible for the deletion through homologous recombination during meiosis. Tetrad (ordered spore) analyses showed that all small spore-derived strains lacked the missing DNA between the two direct repeats that was found in all large spore-derived strains. In addition, many isolates including single-spore isolates were found to be heterokaryotic at the MAT locus, suggesting that such direct-repeats mediated deletion may also occur during mitosis at low frequencies.

Technical Abstract: The fungus Sclerotinia trifoliorum is a pathogen of chickpea and other cool season legumes, and it produces spores of two sizes. The large spores give rise to strains that are self-fertile and produce large and small spores again, but the small spores give rise to strains that are self sterile and cannot produce any spores. This phenomenon is called unidirectional mating type switching. The mechanisms of unidirectional mating type switching in S. trifoliorum are not understood. The present study, comparing DNA sequences in the DNA region (MAT locus) that controls mating and spore production, found four main mating type genes (MAT1-1-5, MAT1-1-1, MAT1-2-1 and MAT1-2-4) in the self-fertile strain derived from large spores. In contrast, this DNA region in the self-sterile strain derived from small spores is 2891-bp shorter than that of the self-fertile strains. Within the missing DNA are the entire MAT1-2-1 and MAT1-2-4 genes. Thus the self-sterile strains do not contain the MAT1-2 genes and that is why they are self-sterile. Meanwhile, a146-bp DNA sequence was found repeated twice and the two repeats flank the deleted region in the self-fertile strains. The two repeat sequences were believed to be responsible for the deletion through homologous recombination during meiosis. Tetrad (ordered spore) analyses showed that all small spore-derived strains lacked the missing DNA between the two direct repeats that was found in all large spore-derived strains. In addition, many isolates including single-spore isolates were found to be heterokaryotic at the MAT locus, suggesting that such direct-repeats mediated deletion may also occur during mitosis at low frequencies.